.TH std::rethrow_if_nested 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::rethrow_if_nested \- std::rethrow_if_nested

.SH Synopsis
   Defined in header <exception>
   template< class E >                    \fI(since C++11)\fP
   void rethrow_if_nested( const E& e );

   If E is not a polymorphic class type, or if std::nested_exception is an inaccessible
   or ambiguous base class of E, there is no effect.

   Otherwise, performs

 if (auto p = dynamic_cast<const std::nested_exception*>(std::addressof(e)))
     p->rethrow_nested();

.SH Parameters

   e - the exception object to rethrow

.SH Return value

   \fI(none)\fP

.SH Notes

   Unlike many related functions, this function is not intended to be called with a
   std::exception_ptr but rather an actual exception reference.

.SH Possible implementation

   namespace details
   {
       template<class E>
       struct can_dynamic_cast
           : std::integral_constant<bool,
                 std::is_polymorphic<E>::value &&
                 (!std::is_base_of<std::nested_exception, E>::value ||
                   std::is_convertible<E*, std::nested_exception*>::value)
             > {};

       template<class T>
       void rethrow_if_nested_impl(const T& e, std::true_type)
       {
           if (auto nep = dynamic_cast<const std::nested_exception*>(std::addressof(e)))
               nep->rethrow_nested();
       }

       template<class T>
       void rethrow_if_nested_impl(const T&, std::false_type) {}
   }

   template<class T>
   void rethrow_if_nested(const T& t)
   {
       details::rethrow_if_nested_impl(t, details::can_dynamic_cast<T>());
   }

.SH Example



   Demonstrates construction and recursion through a nested exception object.


// Run this code

 #include <exception>
 #include <fstream>
 #include <iostream>
 #include <stdexcept>
 #include <string>

 // prints the explanatory string of an exception. If the exception is nested,
 // recurses to print the explanatory of the exception it holds
 void print_exception(const std::exception& e, int level =  0)
 {
     std::cerr << std::string(level, ' ') << "exception: " << e.what() << '\\n';
     try
     {
         std::rethrow_if_nested(e);
     }
     catch (const std::exception& nestedException)
     {
         print_exception(nestedException, level + 1);
     }
     catch (...) {}
 }

 // sample function that catches an exception and wraps it in a nested exception
 void open_file(const std::string& s)
 {
     try
     {
         std::ifstream file(s);
         file.exceptions(std::ios_base::failbit);
     }
     catch (...)
     {
         std::throw_with_nested(std::runtime_error("Couldn't open " + s));
     }
 }

 // sample function that catches an exception and wraps it in a nested exception
 void run()
 {
     try
     {
         open_file("nonexistent.file");
     }
     catch (...)
     {
         std::throw_with_nested(std::runtime_error("run() failed"));
     }
 }

 // runs the sample function above and prints the caught exception
 int main()
 {
     try
     {
         run();
     }
     catch (const std::exception& e)
     {
         print_exception(e);
     }
 }

.SH Possible output:

 exception: run() failed
  exception: Couldn't open nonexistent.file
   exception: basic_ios::clear

.SH See also

   nested_exception  a mixin type to capture and store current exceptions
   \fI(C++11)\fP           \fI(class)\fP
   throw_with_nested throws its argument with std::nested_exception mixed in
   \fI(C++11)\fP           \fI(function template)\fP
